This invention relates generally to color television image pickup devices using a color separation stripe filter for producing a multi-color video signal, and more particularly the present invention relates to a circuit arrangement for such a color TV pickup device.
So-called single-tube color TV cameras with a color separation stripe filter are widely used for producing a multi-color video signal, and such a single-tube color TV camera usually comprises a photoconductive layer at its photoelectric converting portion. The photoconductive layer is used to store therein charges in accordance with the amount of incident light, where the stored charges are then discharged with a scanning electron beam applied thereto so as to obtain an output video signal. The intensity of the scanning electron beam is set to a relatively low value so that a uniform modulation characteristic is obtained throughout the entire target of the pickup tube, and an output signal showing a desired degree of modulation is obtained. As a result of the use of such a relatively low intensity scanning beam, some charges stored in a portion where the amount of incident light is large, cannot be fully discharged. Namely, the modulation degree is deteriorated at a bright portion. Therefore, the output video signal does not accurately represent the brightness of a taken image, especially at a bright portion, deteriorating the quality of an image which will be reproduced on a CRT.
Such an undesirable phenomenon is called insufficient beam phenomenon, and conventionally in order to prevent the modulation degree from being deteriorated by such insufficient beam phenomenon, the intensity of the scanning electron beam is changed in accordance with the amount of light incident on an image pickup tube. However, in a single-tube TV camera, the output video signal is a multi-color video signal which includes high frequency components. Therefore, when the output video signal is used as it is for controlling the intensity of the scanning electron beam, the intensity control is effected with a considerable time delay due to time lag elements inherently existing within the control loop. As a result, such a feedback beam intensity control cannot catch up with the image on the photoelectric converting layer, resulting in unsatisfactory color reproducing images on a CRT.
Since such a time delay is caused by the high-frequency components of the output video signal, beam intensity control for the pickup device of such a single-tube TV camera has hitherto been effected by using only low frequency components of the output video signal. However, when beam intensity feedback control is effected by using the low frequency components of the output video signal, there arises a problem as follows. When the output video signal does not have low frequency components or when the low frequency components are of very low level, the beam intensity feedback control cannot be effectively carried out. As a result, the intensity of the scanning electron beam does not increase in accordance with the brightness of the taking image resulting in deterioration of the reproducing image quality.